The invention relates to a process for the production of 1,2 dichloroethane, hereinafter referred to as EDC, which primarily serves as an intermediate product in the production of monomer vinyl chloride, hereinafter referred to as VCM, which, in turn, is used to produce polyvinyl chloride (PVC). When EDC reacts to form VCM, hydrogen chloride (HCl) is obtained. Hence, EDC is preferably produced from ethylene (C2H4) and chlorine (Cl2) in a manner such as to maintain a balance between the hydrogen chloride (HCl) produced and consumed in the various reactions, as represented by the following reaction equations:Cl2+C2H4→C2H4Cl2 (pure EDC)+180 kJ/Mol  (1)C2H4Cl2 (cracked EDC)→C2H3Cl (VCM)+HCl−71 kJ/Mol  (2)C2H4+2 HCl+½O2→C2H4Cl2 (raw EDC)+H2O+238 kj/Mol  (3)
The process for the production of VCM with an adequate HCl balance —hereinafter referred to as “balanced VCM process”—comprises the following process steps:                direct chlorination in which one portion of the required EDC is produced from ethylene (C2H4) and chlorine (Cl2) in the presence of a homogeneous catalyst and is made available as so-called pure EDC;        oxichlorination in which the remaining portion of the required EDC is produced from ethylene (C2H4), hydrogen chloride (HCl) and oxygen (O2) and made available as so-called raw EDC;        fractionating EDC purification in which the raw EDC, together with the recycle EDC returned from the VCM fractionation step and, optionally, together with the pure EDC is freed from the secondary products formed in the oxichlorination and EDC pyrolysis steps in order to obtain a so-called feed EDC suitable for use in the EDC pyrolysis step; as an option, it is also possible to distil the pure EDC from the direct chlorination step in the heavy-ends column of the EDC distillation section;        EDC pyrolysis in which the feed EDC is thermally cracked, the mixture leaving the reactor, known as cracked gas, consists of VCM, hydrogen chloride (HCl) and non-reacted EDC as well as secondary products;        VCM fractionation in which the desired pure VCM product is separated from the cracked gas while the other essential substances, viz. hydrogen chloride (HCl) and non-reacted EDC contained in the cracked gas, are separately recovered as valuable materials and returned as recycle HCl or recycle EDC to the balanced VCM process.        
In most industrial processes, a circulating stream of EDC reaction product is used as the reaction agent in direct chlorination. This can be accomplished in a loop-type reactor with external or internal circulation. The circulation can also be accomplished in a system with natural or forced circulation. In most cases ferric chloride is used as catalyst and in addition, sodium chloride which is able to inhibit the formation of heavy ends, may be admixed as an additive.
The state of the art as regards direct chlorination is, for instance, described in DE 199 10 964 A1. The process according to DE 199 10 964 A1 aims at suppressing side reactions, especially the continuation of the chlorination process of EDC to 1,1,2 trichloroethane, by making most of the chlorination reaction take place in the homogeneous liquid phase. The ethylene, which is less readily soluble in EDC than chlorine, is completely dissolved in the main stream of the circulating EDC reaction fluid in a co-current bubble column. The chlorine, which is more readily soluble in EDC than ethylene, is dissolved in a supercooled EDC part-stream and the resulting solution of chlorine in EDC is fed to the circulating main stream which already contains the dissolved ethylene.
Reaction (1), as a rule, is run with a slight ethylene surplus in order to avoid in any case any corrosion in the reaction system, the formation of secondary products at the end of the direct chlorination reaction and other problems associated with the treatment of chlorine-bearing outlet streams. Chlorine and ethylene are fed to the reactor by means of a ratio controller, the control variable being the ethylene content of the reaction outlet stream. In this case the aim is always to minimise the ethylene surplus at the reactor outlet to the extent possible in order to preclude too large an ethylene loss.
It was also found that reaction (1) produced a particularly high rate of secondary products when it was run as liquid phase reaction as shown in WO 03/070673 A1. This necessitates that ethylene is completely dissolved in the reaction tube prior to adding chlorine. The small gas bubbles generated by the gas distributor slowly grow by coalescence when travelling along this section and they finally reach a constant equilibrium size as a result of coalescence and decomposition activities. This impact adversely affects the mass transfer as the enlargement of the bubble diameter at a given total gas volume reduces the surface area available for mass transfer.
The kinetics of reaction (1) which takes place in the adjacent reaction zone in a largely homogeneous manner follows the velocity principle of the second order, hence at a very high velocity. The reaction velocity sharply drops at the end of the reaction zone when the ethylene and chlorine concentrations diminish gradually.
The overlapping effects that affect the ethylene solution behaviour, the reaction itself and the start of boiling clearly govern the sizing of a state-of-the-art boiling reactor and render a subsequent increase in capacity more difficult.